Tapered or bevel gears of hypoid tooth profile are primarily used in gears for automotive vehicles and helicopters. They are prepared from high-alloy steels in a soft state on special gear cutting machines having profiled cutting tools, and the cutting is carried out by the rolling or hobbing method. Afterwards the gears are hardened.
Due to the inavoidable hardening distortions they loose their original accuracy. In order to make them suitable again for their use in vehicle gears, they are lapped in pairs on special lapping machines. During lapping also the most favorable mounting position for each wheel pair is determined. During mounting of the gear the mounting of the wheel pairs must be adjusted to this present position by means of calibrated washers and/or particular bearing bushes.
One would prefer to avoid this labor-intense handling and also desire to have higher accuracies as well as surfaces of "grinding quality". Unfortunately bevel gears of hypoid tooth profile have a very complex tooth geometry. The flank lines in the rolling plane may be spirals, cycloids, hypoids, circular arcs or involutes. The narrow curvatures of the teeth and the narrow tooth gaps permit only the use of grinding tools of the size approximately of a pencil tip or, exceptionally, also thin-walled cup wheels. These grinding tools exclude already the possibility of grinding economically precut or preforged bevel gears having dimensions that are increasd by the so-called grinding allowance.
From tool making it's known to machine complex three-dimensional workpieces by means of spark erosion and/or by the electrochemical method. as generally known, the subsequent hardening and therefore the undesired hardening distortions are avoided. The most recent state of the art on spark erosion is described in leaflet EDM 3374, Robofil/Roboform, of Charmilles Technologies S.A., 109 rue de Lyon, Geneva (Switzerland), dated May 1984. On pages 12 and 13 the possible working forms of the immersion erroding machine are represented and briefly described.
For machining bevel gears of hypoid tooth profile by means of spark erosion, the "spiral-shaped machining of threads and spiral-shaped recesses" comes closest to it. On this kind of machining the workpiece is immersed, with a relative helical or screwmotion between it and the electrode, into the later which has the negative shape of the workpiece. With this kind only such workpieces can be machined which correspond to the following conditions:
no contact must occur during immersion, and PA1 one must never fall below, along the entire spiral arc, before the tecnhologically necessary working gap width.
With this kind of mahcining a cylindrical or tapered thread e.g. can be made only if the thread pitch is constant. If the thread pitch varies more along the entire spiral than does the technologically necessary width of the working gap between the electrode and the workpiece, interlocking occurs during immersion of the workpiece into the electrode. If the thread pitch along the entire thread spiral varies as little as the technologically necessary working gap width, the thread will be partially damaged during immersion of the workpiece into the electrode. It is to be noted that in all three cases the workpiece and the electrode are equidistant in their final position.
Gear wheels of hypoid tooth profile are complex three-dimensional bodies. The pitch of the teeth, the tooth width and the flank normals vary along the tooth width according to higher orders. Therefore they will be briefly described in a practical manner with the pitch angle .delta., with the cone distance R, with the tooth width b, the segment angle in function of the tooth width .beta.=F(b), the module m and with the normal pressure angle a.
The additional data and characteristics of the bevel gears will be determined by the generating machines. The required vibration and noise tolerances call for maintaining the geometrical accuracies within the range of a few thousands of a millimeter.
The machining of bevel gears of hypoid tooth profile by immersion into the electrode which has the negative shape of the gear wheel is not possible although the electrode would be equidistant to the bevel gear in its final position. Depending upon the variation of the above mentioned parameter of the bevel gear of hypoid tooth profile, the tooth flanks, during insertion of the workpiece into the electrode, will more or less become partially damaged along the tooth width, and in more unfavorable cases even interlocking occurs.
Also it is not possible and could, in addition, not be economical to use an electrode of regularly widened working gap and planetary movements of known manner, firstly due to the very close variation of the surface normal and secondly due to the concentration of the erosion front onto a very small area. The invention tends to overcome these deficiencies.